The present invention relates to automatic light level controls for TV camera systems and more particularly to an improved automatic light control circuit that provides a constant effective exposure to the image tube photocathode of an active TV camera system.
Operation of TV camera systems under conditions of variable scene illumination requires some control of camera sensitivity, specifically that of the image tube, to protect the tube from excessive photo-emission and photoelectron flow at high light levels and to provide generally a constant video output signal. Such sensitivity control may be manual but for the most part have been automatic and in the form of electronic circuits commonly called automatic light control (ALC) circuits.
In conventional implementations of ALC circuitry, a scanned video output signal of the image tube is sampled at one or more points along a video amplifier chain. These video samples are then compared to a reference value and used to adjust certain parameters including optical transmission via an adjustable iris opening, sensor gain, and video amplification thereby controlling camera sensitivity.
While existing ALC circuitry have been successful in providing an effective means of sensitivity control for the image tubes of TV camera systems, such circuitry have been limited in their response time to several vertical scan intervals because of the sampled data nature of the video signal which, at any point in time, represents the time integral of scene illumination multiplied by system sensitivity for the preceding vertical interval. A lack of a fast-responding sensitivity control of the image tube has been particularly disadvantageous in applications of active TV cameras wherein a pulsed light source confined to either the vertical or horizontal retrace interval is utilized to provide the scene illumination for the camera.